This invention relates generally to the locating of electrical shorts and more particularly to a method of locating electrical shorts in multi-layer ceramic electronic substrates.
In the manufacture of very large scale integrated circuits for use in high speed computers and microprocessors, a multi-layer ceramic substrate is frequently utilized to support and interconnect many integrated circuit components. Each such substrate is formed from a large number of individual unfired screened ceramic sheets containing a conductive pattern and many conductive via holes. The via holes are utilized to connect such patterns with patterns on other sheets in the substrate, particularly with certain sheets utilized to supply required voltage levels to circuit components, known as voltage planes, and to connect such patterns and voltage planes with integrated circuit components mounted on the substrate. The individual sheets are stacked to form a laminate and baked in a sintering furnace to form a ceramic brick. A high density interconnect layer is then deposited on the ceramic brick substrate and electrically connected to the substrate. The layer of high density interconnect is formed from a plurality of thin film layers each having a thickness about one fifth the thickness of a single ceramic sheet. These thin film layers perform the same function as the layers forming the ceramic brick in much less space and rely on the ceramic brick for structural support. The top surface of the substrate includes a large number of electrical contact pads. A desired array of integrated circuit components may then be mounted on the top surface of the substrate and be interconnected in a desired manner.
During this manufacturing process short circuits are occasionally created between circuit paths in the high density interconnect portion of the substrate. The existence of unwanted short circuits can be detected using known electrical test procedures. The cost of manufacturing such multi-layer ceramic substrates is very high and thus it is highly desirable to repair such short circuit defects if possible. Heretofore, however, no effective technique has been available to locate such short circuit defects so that they may be repaired. Optical inspection techniques using a microscope have proved to be quite time consuming, lasting up to fifteen hours or more, and even then without success in some instances. Heat sensitive photography has also been used by delivering a current to the short circuit and attempting to locate the short circuit through the use of infrared photography. Such attempts have suffered from the disadvantage that the shorts would frequently melt before they could be located, thus rendering the substrate useless since the melted residue of the short itself creates an unacceptable defect.